The invention relates to a device and a method for the non-destructive testing of passive components.
Quality control is an important aspect of the manufacture of components. Components to be tested are subjected to test loads for an appropriate period of time in order to detect defects. The electrical quantities that can be measured at the contacts of the components are used for evaluation. Defects inside components can also be detected by using X-rays or ultrasound.
U.S. Pat. No. 5,510,719 describes a method of testing capacitors. To this end, a test voltage is applied to a capacitor which is loaded for a given period of time at a relevant temperature. A residual voltage, which lies below a threshold voltage, is used as a criterion for quality evaluation. This residual voltage is measured at the contacts of the capacitor.
According to the described method an electrical response on the contacts of the component to be tested is checked.
The methods that are known thus far have the drawback that carrying out such a test usually requires a significant amount of work. Moreover, the known methods involve the risk that components of inferior quality are completely destroyed by the test.
A further, external test signal is applied to the component for the testing by means of X-rays or ultrasound. This requires an additional implementation of, for example an X-ray source or an ultrasound source.
Therefore, it is an object of the invention to provide a device and a method offering a simple, reliable evaluation of the failure tolerance of the component to be tested.
This object is achieved by measuring an acoustic emission by a component which is subjected to an electrical test signal.
In the context of the normal test an electrical test signal is applied to the contacts of the component to be tested. This test is intended to test, for example the functional basic properties, such as the resistance or capacitance, of the component. Internal defects of the component convert such properties into oscillations which can be measured as an acoustic emission. The frequency spectrum of faulty components deviates significantly from the frequency spectrum of high-quality components. A decision as regards the quality of the tested component is taken on the basis of this difference.
The acoustic emission can be measured by means of a microphone which is arranged in the direct vicinity of the component. The measured signal is applied to an evaluation unit in which a comparison can be performed by way of spectral analysis, the comparison result being used, for example, to control a sorting machine.
Depending on the type of component, a test voltage or a test current is applied to the terminals of the components to be tested.
It is particularly advantageous to use a test signal in the form of a pulse voltage. The electrical field then arising exerts forces on stationary and moving charge carriers. This effect is particularly pronounced in the case of ferroelectric materials such as used in a variety of electrical components, for example, in non-linear resistors or ferroelectric capacitors. Ferroelectric materials have a high dielectric constant and their response to external electrical fields takes the form of a shift of domain walls. In turn this shift induces mechanical stresses in the material which, when they exceed a threshold value, may cause cracks and/or delaminations. The electrical properties of the components can then still be within a specific range. Latent damage in the form of cracks or delaminations, however, represents a potential risk for premature failure of the component. For example, the breakdown strength is then usually reduced. When exposed to electrical, mechanical or atmospheric stress (temperature, moisture) for a prolonged period of time, faults occur (for example, short-circuits).
In the case of internal defects of the components, the relevant test voltage of suitable shape, magnitude and duration induces the formation of cracks in faulty components. The resultant acoustic emission enables the detection of the cracking and ultimately of a component of inferior quality. A test voltage in the form of a direct voltage or an alternating voltage can be used as necessary.
In the case of passive components, consisting of ferroelectric or ferromagnetic or electrostrictive or magnetostrictive materials, the method according to the invention offers a reliable evaluation of the quality of the component to be tested.
The device according to the invention and the application of such a method are particularly suitable for the testing of resistors and capacitors.
Because the components are normally tested by means of test voltages or test currents in automatic testing devices, the device and the method according to the invention can be readily implemented in the quality control or the test procedure for the output check as well as for the input check.
It is an advantage of the invention that an automatic testing device need not be additionally equipped with special apparatus for controlling the samples. Moreover, the acoustically measurable emission can be simply detected. For the testing of components where a partial discharge, impedance spectra or resonance shifts are measured it has been found that the measurement of an electrical response on the terminals of the component is significantly more intricate.
The means for carrying out this test can be simply integrated in existing automatic testing devices. When X-rays or ultrasound are used for fault detection, such a simple implementation is not possible. The non-destructive testing constitutes a further advantage. Only components of inferior quality respond by cracking or delamination when subjected to a pulse-like test signal. The properties of components of high quality are not influenced by such loading.